Abrasion defects are defects that are individually of the order of several micrometers in width and sub-micrometer to micrometer in depth. They can be formed as a group and are sometimes referred to as ‘scuff marks’ and are commonly observed on the front and back sides of photographic films, particularly those used with printed circuit board (PCB) photomasks. They are caused by low-pressure sliding contact of dirt particles or rough surfaces in contact with imaging products. The defect can be purely cosmetic in that the usage of the product is not hindered but is totally unacceptable as it would give the impression of poor workmanship. Image information is contained within photographic films in the form of developed silver, whilst for other imaging elements it could be in the form of a diazo compound. If the scuff mark is severe, some of the image information can be lost making the medium useless.
Scratches are defects that also are micrometers in width but tend to be deeper than a scuff mark and tend to be present singly. They can also be formed on either side of an imaging medium, in particularly on PCB photomasks, and are caused by higher-pressure sliding or digging contact of objects when in contact with imaging products. Again this type of defect is particularly serious if part of the information contained within the image is lost as the medium now becomes useless.
A chucking abrasion is similar to the scratch described above but the dirt particles that cause the defect tend to wander over a small area of film in a random walk pattern. Such particles cause this type of damage when they are included within a stack of film with the films sliding about when the box of film is transported without vacuum packaging. Since the particle may wander over its own path, this defect is also termed a repetitive scratch defect.
Photoabrasion is a defect that occurs to a photographic film prior to processing or during processing. A scratch or an abrasion defect injures the film to the extent that the silver halide crystals become developable without the action of light. This defect shows up as a line or series of lines of developed silver in areas where no image was expected. This can make the photographic film useless for its intended purpose.
Thus it will be seen that abrasion-resistance and scratch-resistance protective coatings on the front or back or both sides of an imaging element are commonly required.
Black-and-white high contrast silver halide materials are used widely as originals for optical contact copying onto other photosensitive materials. For example in the printing industry, page separations are exposed by imagesetter onto film, which is then copied (exposed) onto printing plates by ultraviolet (UV) contact exposure. The exposed printing plate is then processed to produce an ink-receptive image for printing on a press. Sometimes pages are physically assembled by cutting and pasting images and text from various sources and the assembled page may then be copied by camera onto another sheet of film. A scratch defect in this film will remove image, leading to an unusable printing plate, and a photoabrasion defect will lead to an unwanted line on the plate.
Another example of this type of process is the manufacture of PCBs, wherein electronic circuit track layouts are exposed by photoplotter onto film intermediates, called phototools or photomasks. The photomask is positioned immediately adjacent to a copper-clad, resist-covered PCB substrate on a contact-copying frame. To improve the intimacy of contact it is normal practice to evacuate air from the contact frame. Incident radiation such as ultraviolet light is provided by a source within the contact frame and is received by the photomask and transmitted directly to the photoresist layer through openings in the mask. The mask is a photographic film and openings in the mask correspond to minimum density regions of the image on the film.
The transmitted radiation causes a change in the properties of the photoresist, e.g. a hardening or a solubilising, such that areas exposed to the radiation are physically different from those that have been hidden by the mask layer. After exposure, the photoresist is processed to remove it from areas where it is desired to etch away the copper, such that the resultant structure has regions of copper exposed and regions concealed by photoresist. The exposed areas of the copper are then etched and, after etching, the remaining photoresist is removed from the PCB to reveal the track pattern.
PCB photomasks have the most demanding of durability requirements since, unlike most other photographic films, the developed image is used many times in the imaging of the photoresist of a PCB. For instance, an imagesetter film once processed may be used once to produce a printing plate, whereas a PCB photomask may be used one thousand times in its lifetime, but due to scratch defects this may be limited to a more typical value of one hundred times. Photomasks are therefore generally protected by a much thicker outermost layer than are most imaging elements, i.e. of the order of 1.5 μm, which can lead to inefficient use of lubricant.
A variety of methods have been investigated to overcome the various injuries to which an imaging element, and particularly a photographic film, may be subjected. U.S. Pat. No. 3,121,060 describes the use of waxy esters of higher fatty alcohols and higher fatty acids, typified by modified sperm oils, as lubricants for photographic film in its unexposed form to reduce friction when passed through film magazines and to impart more permanent lubrication. There is no disclosure of the use of these esters for preventing scratches or abrasion.
In U.S. Pat. No. 6,346,369 there is disclosed an imaging element which includes a scratch-resistant outermost layer either overlying the imaging layer or on the side opposite the imaging layer, which is composed of a ductile polymer of defined physical properties. The element is disclosed to have resistance to the formation of permanent scratches that may arise during manufacture and use of the imaging product. Silicone, fatty acid and especially carnauba wax lubricants are disclosed for overcoating the scratch-resistant layer. There is no disclosure therein of the element providing resistance to abrasion.
JP 63005339 discloses a photosensitive material comprising layers on both sides of the support wherein the uppermost layers each contain at least one of an organopolysiloxane, a fluorine-containing compound and a nonionic surfactant containing a polyoxyethylene unit, with all three of the above in at least one of those layers. The expressed object is to control electrostatic charge. JP 63008642 describes a photosensitive material with good antistatic properties which includes an outermost layer containing an organic polysiloxane and a nonionic surfactant, the material having a good lubricating effect.
JP 01107255 describes a material wherein a silicone-based compound is contained in at least one layer other than the outermost layer, and the outermost layer contains a different lubricant, which may however be also be a silicone compound, the material reducing sliding friction. JP 03132754 discloses a photosensitive material having good lubricating properties and scratch resistance, capable of preventing uneven drying after processing, by incorporation of a higher fatty acid amide or ester and a silicone compound both together into the outermost surface layer of a backing layer. There is no disclosure of an abrasion-resistant effect being obtained thereby.
U.S. Pat. Nos. 5,019,491, 5,702,864 and 6,482,580 all describe the use of a siloxane in an outermost layer of an imaging element, together with another lubricant or similar addendum in that same layer.
None of the above discloses the combination in an imaging element of a lubricant providing abrasion-resistance in the outermost layer, with a lubricant in the interlayer providing scratch-resistance.